X-linked Retinitis-Pigmentosa (XLRP) is the most severe form of the RP subtypes. It is characterized by an early onset of night blindness (within the first two decades), progressing steadily and culminating in severe visual impairment (usually complete blindness) by the fourth decade. The frequency of XLRP is estimated at about 1/10,000 representing about 20% of the hereditary forms. The genes responsible for XLRP map on the short arm of the X-chromosome, Xp11.3 (RP2) and Xp21.1 (RP3). In spite of numerous genetic linkage data, the carrier detection and prenatal diagnosis of XLRP remains problematic. The RFLP probes available today are not suitable for this purpose. Funduscopic examination and electroretinography (ERG) can detect only a portion of carriers only when tested in the later stage of life. Carrier detection is especially difficult for young daughters of female carriers who carry the risk of transmitting the disease to their sons and yet do not exhibit any retinal abnormality. And there is no test available for prenatal diagnosis. It is, therefore, extremely important to develop new RFLP probes linked closely to the RP gene(s), which will be able to predict the probes linked closely to the RP gene(s), which will be able to predict the carrier status and diagnose RP in prenates, in a manner similar to CF, RB, CGD and DMD. The long term objective of this project is to isolate the XLRP gene(s) and to understand the biochemical basis of the disease. The specific aim of this proposal is to establish a long range physical map around the RP2 (DXS7) and the RP3 (OTC) loci, using rare cutter restriction endonucleases and the Pulse Field Gel Electrophoresis (PFGE). The CHEF (Clamped Homogeneous Electrical Field) gel system will be employed for the analysis and the initial characterization will be performed on the lymphoblastoid cell line, 48xxxx(GM416), enriched for the X-chromosome. The physical mapping will be extended to lymphoblastoid cell lines, derived from a XLRP male and, those having Xp microdeletions and RP phenotypes. These physical mapping data will then be utilized to construct new probes for the diagnosis and eventual cloning of the XLRP gene(s).